The objective of this proposal is to comprehensively characterize cytokine mRNA and protein expression in the lymphoid tissues of FIV-infected cats and attempt to modulate the cytokine response with anti-cytokine antibodies and recombinant Listeria monocytogenes. Hypothesis: Inappropriate cytokine production contributes to the ineffective immune response to FIV, the patho- genesis of FIV, and the susceptibility of FIV infected cats to opportunistic infections. Modulation of cytokines may be useful to study the efficacy of therapeutic interventions and vaccine strategies, as well as the pathogenesis of FIV. Specific Aim 1: to determine the effect of FIV infection on cytokine mRNA and protein production in tissues of cats, multiple lymphoid compartments will be evaluated by QC-RT-PCR to quantify tissue production of cytokine mRNA, by in situ hybridization to localize mRNA within the tissue structure, and by immunocytochemistry, ELIspot, and flow cytometry to determine cytokine protein production within specific cell phenotypes. Specific Aim 2: to determine the significance of IL2, IL4, IL10, IL12, TNF-alpha and IFN-gamma in the immunopathogenesis of FIV, anti-IL2, anti-IL4, anti-IL10, anti-IL12p40, anti-TNF-alpha or anti-IFN-gamma monoclonal and polyclonal antibodies will be administered to acutely FIV infected cats. The effects of cytokine inhibition on viral replication, immune response, and disease progression will be determined. Specific Aim 3: To determine if a strong cell-mediated immunity to FIV is protective, SPF cats will be vaccinated with live recombinant biological vaccine vector; FIVenv- and/or FIVgag- expressing Listeria monocytogenes. Cats will then be challenged with homologous and heterologous FIV and the immune response and cytokine profile associated with either protective or non- protective immunity will be evaluated. Significance: Lentivirus- associated immunodeficiency is an expanding global problem affecting humans and cats, however the immunopathogenesis is incompletely understood and current therapies are limited. Understanding the role of the cytokines in the pathogenesis, therapy, and vaccination in FIV infection will have direct implications for HIV infection.